linux-kernel - Re: [PATCH 03/32 RESEND] nohz: Drop useless ts->inidle check before rearming the tick

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Message-ID: <20110829165850.GA9748@somewhere.redhat.com>
Date:	Mon, 29 Aug 2011 18:58:53 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 03/32 RESEND] nohz: Drop useless ts->inidle check before
 rearming the tick

On Mon, Aug 29, 2011 at 04:23:15PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > We only need to check if we have ts->stopped to ensure the tick
> > was stopped and we want to re-enable it. Checking ts->inidle
> > there is useless. 
> 
> /me goes la-la-la-la... 
> 
> It would so help poor little me who hasn't stared at this code in detail
> for the past several days and is thus horridly confused if you'd expand
> your reasoning somewhat.

Sorry, I'm no big fan of writing changelogs and sometimes the lack
it's unfortunately visible :)$

It needs to be refactored due to the previous patch beeing broken.
But the rationale, indeed missing here, is that if you have ts->stopped
then you have ts->inidle. Once you entered tick_nohz_stop_sched_tick()
you have ts->inidle set and only once you reached that step the tick
can be stopped, so the following check:

	if (!ts->inidle || !ts->tick_stopped)

can be summed up with:

	if (!ts->tick_stopped) {

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